3.10: Nomenclature of Ionic Compounds

The simplest ionic compounds consist of a single type of cation associated with a single type of anion. Nomenclature for these compounds is trivial; the cation is named first, followed by the anion. If the anion is a single element, the suffix ide is added to the root name of the element.

When you are constructing names for ionic compounds, you do not use “multipliers” to indicate how many cations or anions are present in the compound. For example NaI is named sodium iodide; Na2S is named sodium sulfide; CaCl2 is named calcium chloride. The chemist reading the name is assumed to have sufficient knowledge to pair the elements properly based on their common valence states. There are exceptions to this simple nomenclature, however. Many transition metals exist as more than one type of cation. Thus, iron exists as Fe2+ and Fe3+ cations (they are referred to as “oxidation states”, and will be covered in detail in Chapter 5). When you are naming an ionic compound containing iron, it is necessary to indicate which oxidation state the metal has. For metals, the oxidation state is the same as the charge. Thus Fe2+ in a compound with chloride would have a formula FeCl2 and would be named iron (II) chloride, with the oxidation state (the charge on the iron) appearing as a Roman numeral in parenthesis after the cation. The cation Fe3+ paired with oxygen would have the formula Fe2O3 and would have the name iron (III) oxide.

The procedure for naming ionic compounds contain polyatomic ions is identical to that described above for simple ions. Thus, CaCO3 is named calcium carbonate; Na2SO4 is named sodium sulfate; (NH4)2HPO4 (a compound with two polyatomic ions) is named ammonium hydrogen phosphate; and Pb2+ paired with SO42-, PbSO4 is named lead (II) sulfate.

Example \(\PageIndex{1}\):

Write a correct chemical formula for each of the following ionic compounds:

Calcium bromide

Aluminum oxide

Copper (II) chloride

Iron (III) oxide

Solution

Calcium is 2+, bromide is 1-; CaBr2.

Aluminum is 3+, oxide is 2-; Al2O3.

From the oxidation state that is given, copper is 2+, chloride is 1-; CuCl2.

From the oxidation state, iron is 3+, oxide is 2-; Fe2O3.

Example \(\PageIndex{1}\):

Write a proper chemical name for each of the following ionic compounds:

Li2S

CaO

NiCl2

FeO

Solution

We don’t use multipliers, so this is simply lithium sulfide.

This is simply calcium oxide.

We don’t have to specify an oxidation state for nickel, so this is nickel chloride.

We must specify that iron is 2+ in this compound; iron (II) oxide.

Exercise \(\PageIndex{1}\)

Write a correct chemical formula for each of the following ionic compounds:

Sodium phosphide

Iron (II) nitrite

Calcium hydrogen phosphate

Chromium (III) oxide

Exercise \(\PageIndex{1}\)

Write a proper chemical name for each of the following ionic compounds:

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